In preferred embodiments, a heating coil is provided that includes at least a set of heating conductors, one of which is arranged at a side of a work and the other one is arranged at an opposite side of the work. In addition, a plurality of connecting conductors are provided to connect an end of one of the heating conductors with an end of the other heating conductor, wherein each of the heating conductors is connected with at least two connecting conductors. The connecting conductors extend from an end of each of the heating conductor in a different direction from each other. In the preferred embodiments, the heating coil has a simple structure that enables an even heat treatment to a work in a width direction.